A lot of effort has already been done in order to tune the ion-dynamics (i.e. battery performance) in LFPO. Here, nanostructuring as well as surface coatings (with carbon as well as with other elements and compounds) [1]} are among the most common approaches. Such systems have also been studied systematically using both neutron scattering [2]} and \(\mu ^+\) SR [3]}. When it comes to the tuning of low-temperature magnetic properties, much less detailed studies are to be found in the published literature. Indeed, chemical substitution in LFPO is possible, yielding e.g. NaFePO\(_4\) , which display a slightly larger unit cell [4]} and also an expected reduction in the magnetic ordering temperature (\(T_{\rm N}\) ) by a few Kelvin (Fe-Fe distance is larger). Of course, nanostructuring and surface coatings could also induce interesting effects on the magnetic properties, however, other more direct and 'cleaner' approaches could be employed first. In fact, a magnetic field dependence of the AFM magnons in LFPO has recently been presented [5]}. Another very common method is to apply hydrostatic pressure, thereby compressing the crystallographic lattice, and investigate how this affects the spin correlations in a material. Combining the penetrating properties of muons, the ultra-high sensitivity of \(\mu ^+\) SR to detect changes in spin order/dynamics, along with hydrostatic pressure is a very powerful approach [6]}, [7]}, [8]}, [9]}. In this study we have used a similar protocol to conduct the first investigation of the local magnetic properties of LFPO under hydrostatic pressure.
